This invention relates to acoustic logging of boreholes and more particularly to a borehole logging system employing a borehole televiewer with automatic level control for the logging of the characteristics of the material forming the walls of the borehole.
In U.S. Pat. Nos. 3,371,313 to Zemanek; 3,485,317 to Patel; 3,503,038 to Baldwin; 3,518,679 to Baldwin et al; and 3,718,204 to Groenendyke there are disclosed methods of and apparatus for scanning the walls of a borehole with acoustic energy. In these patents a borehole televiewer logging tool employs a transmitter and a receiver of acoustic energy which are rotated within the borehole. The transmitter is cyclically energized to provide a beam of acoustic energy pulses for scanning the walls of the borehole. Reflected acoustic pulses are received by the receiver between transmitted acoustic pulses and are converted to reflection signals for recording on an electron beam display device. A sweep signal is generated each time the acoustic energy beam is rotated through a 360.degree. scanning pattern. Such sweep signal is applied to the horizontal deflection plates of the display device to sweep an electron beam horizontally across the face of the display device. The reflection signals are applied to the Z-axis of the display device to intensity modulate the electron beam as the beam is swept across the face of the display device to provide a picture which is a function of the time or distance from the transmitter and receiver to the wall of the borehole and of the density of the borehole wall.
In both the above-described patents the combination of transducer rotation along with vertical movement of the borehole televiewer logging tool along the length of the borehole results in a continuous spiral of the borehole wall being scanned. The. resulting display is a picture of the density of the material forming the walls of the borehole at different depth points. This scanning of the borehole wall permits the determination of the actual configuration of the borehole. In addition, it permits the determination of anomalies which may exist at different depths in the borehole. For example, these anomalies may be a fault or a fracture in the formations traversed by the borehole.
In the aforementioned patents, the transmitter is an acoustic transducer cyclically energized by a suitable voltage pulser. As shown in the aforementioned U.S. Pat. Nos. 3,485,317; 3,503,038 and 3,518,679, a particularly suitable pulser employs an oscillator, a rectifier and a capacitor. The capacitor is charged from a D.C. voltage supply. Each oscillator pulse causes the rectifier to conduct, thereby allowing the capacitor to discharge. A voltage generated by the discharge of the capacitor is coupled through a transformer to excite the transducer to generate an acoustic pulse.
Also in such patents, the acoustic reflections received by the transducer from the borehole walls are amplified and passed through a detector for producing an envelope of the amplified reflection signals. This envelope is applied to the Z-axis of a cathode ray tube display device.
Acoustic noise can be a problem when using the above described borehole logging systems since it is generally greater than electronic noise. It is therefore important to maintain as low an acoustic noise level as possible. In the aforementioned U.S. Pat. No. 3,518,679, the acoustic noise present in the detected reflection signals is lowered by use of a variable gain amplifier. However, in such patent the acoustic noise in the reflection signal is not reduced relative to the amplitude of the reflection signals but is merely a function of the gain selected. It is a specific object of the present invention to control the level of acoustic noise relative to the reflection signal irrespective of the gain to be applied to the reflection signal.